1. Field of the Invention
The invention is directed to a single mode optical waveguide fiber for use in telecommunication systems. More particularly, the waveguide fiber reduces non-linear dispersion effects and provides an expanded window of operating wavelengths.
2. Technical Background
A waveguide having large effective area reduces non-linear optical effects, including self phase modulation, four wave mixing, cross phase modulation, and non-linear scattering processes, all of which can cause degradation of signals in high power systems. In general, a waveguide fiber having a segmented core can provide a large effective area while maintaining other fiber properties within desired ranges. A core having multiple segments, each characterized by a refractive index profile, a relative index, and a radius, has a flexibility of design sufficient to meet an extended list of desired functional properties.
In U.S. Pat. No. 5,781,684, incorporated herein by reference as though fully set forth in its entirety, there is disclosed and described a segmented core waveguide fiber having a large effective area. A feature of the segmented core of that waveguide fiber is that at least one of the segments has a negative relative refractive index.
The present application discloses and describes segmented core waveguide fibers, in which at least one segment has a negative relative index, that provide a unique set of functional properties.
Definitions
The following definitions are in accord with common usage in the art.
The radii of the segments of the core are defined in terms of the index of refraction of the material of which the segment is made. A particular segment has a first and a last refractive index point. A central segment has an inner radius of zero because the first point of the segment is on the centerline. The outer radius of the central segment is the radius drawn from the waveguide centerline to the last point of the refractive index of the central segment. For a segment having a first point away from the centerline, the radius from the waveguide centerline to the location of this first refractive index point is the inner radius of that segment. Likewise, the radius from the waveguide centerline to the location of the last refractive index point of the segment is the outer radius of that segment. PA1 The effective area is generally defined as, EQU A.sub.eff =2.pi.(.function.E.sup.2 rdr).sup.2 /(.function.E.sup.4 rdr), PA1 where the integration limits are 0 to .infin., and E is the electric field associated with the propagated light. An effective diameter, D.sub.eef, may be defined as, PA1 The relative index of a segment, .DELTA.%, as used herein, is defined by the equation, EQU .DELTA.%=100.times.(n.sub.i -n.sub.c)/n.sub.c, PA1 where n.sub.i is the maximum refractive index of the index profile segment denoted as i, and n.sub.c, the reference refractive index, is taken to be the minimum index of the clad layer. Every point in a segment has an associated relative index. The maximum relative index is used to conveniently characterize a segment whose general shape is known. PA1 The term refractive index profile or simply index profile is the relation between .DELTA.% or refractive index and radius over a selected segment of the core. The term alpha profile refers to a refractive index profile that may be expressed by the equation, EQU n(r)=n.sub.0 (1-.DELTA.[r/a].sup..alpha.), PA1 where r is core radius, .DELTA. is defined above, a is the last point in the profile segment, the value of r at the first point of the .alpha.-profile is chosen in accord with the location of the first point of the profile segment, and .alpha. is an exponent which defines the profile shape. Other index profiles include a step index, a trapezoidal index and a rounded step index, in which the rounding is usually due to dopant diffusion in regions of rapid refractive index change. PA1 Total dispersion is defined as the algebraic sum of waveguide dispersion and material dispersion. Total dispersion is sometimes called chromatic dispersion in the art. The units of total dispersion are ps/nm-km. PA1 The bend resistance of a waveguide fiber is expressed as induced attenuation under prescribed test conditions. A bend test referenced herein is the pin array bend test that is used to compare relative resistance of waveguide fiber to bending. To perform this test, attenuation loss is measured for a waveguide fiber with essentially no induced bending loss. The waveguide fiber is then woven in a serpentine path through the pin array and attenuation again measured. The loss induced by bending is the difference between the two measured attenuation values. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. During testing, sufficient tension is applied to make the serpentine woven waveguide fiber conform to the portions of the pin surface at which there is contact between fiber and pin.
The segment radii may be conveniently defined in a number of ways. In this application, radii are defined in accord with the figures, described in detail below.
The definitions of segment radius and refractive index, used to describe refractive index profile, in no way limit the invention. Definitions are given herein because in carrying out model calculations, the definitions must be used consistently. The model calculations set forth in the tables below are made using the geometrical definitions labeled in the figures and described in the detailed description.
A.sub.eff =.pi.(D.sub.eff /2).sup.2.
Another bend test referenced herein is the lateral load test. In this test a prescribed length of waveguide fiber is placed between two flat plates. A #70 wire mesh is attached to one of the plates. A known length of waveguide fiber is sandwiched between the plates and a reference attenuation is measured while the plates are pressed together with a force of 30 newtons. A 70 newton force is then applied to the plates and the increase in attenuation in dB/m is measured. This increase in attenuation is the lateral load attenuation of the waveguide.